Microstructure with movable mass

ABSTRACT

A microstructure includes a mass; a base member in which the mass is movably contained. The mass includes a surface, which is exposed out of the base member, and a stopper wire, which is arranged above the surface of the mass so as to inhibit over move of the mass.

TECHNICAL FIELD

The present invention relates generally to a microstructure with amovable mass (or moving mass). The present invention particularlyrelates to an accelerometer, which detects physical movement of a mass.

BACKGROUND ART

There are a variety of types of sensors using a movable mass (movingmass). For example, there is an inertial sensor such as an accelerometerand an angular accelerometer (vibration gyroscope).

An accelerometer, which detects an acceleration of a vehicle, generallyuses a piezoresistive effect. According to such a sensor, for example, abox shape of seismic mass (i.e. moving mass) is contained in a cavity ofa silicon base member (fixed frame). The movable mass is suspended bybeams on which a piezoresistance are formed, so that a stress is appliedto the piezoresistance in response to movement of the mass. Thevariation of stress applied to the piezoresistance is detected as avariation of resistance. Such technology can be used for cruise controlof vehicles.

The above-described mass is required to move freely, however, if themass over moves, the sensor may be broken or damaged. According to aninvention shown in Japanese Patent Publication Kokoku H5-71448, a glassstopper is used for inhibiting over-move of the mass.

However, a glass stopper is arranged above the mass, so that thethickness of the sensor is increased. Further, fabrication steps arecomplicated and fabrication costs are increased. In addition, a stressgenerated between a glass and silicon member may affect negatively tocharacteristics of the sensor.

DISCLOSURE OF INVENTION

Accordingly, it is an object of the present invention to provide amicrostructure, in which over-move of a mass can be inhibited withoutsignificant increase of thickness. Another object of the presentinvention is to provide a microstructure, in which an over-moveinhibiting mechanism is realized without negative affect tocharacteristics thereof and complication or increase of costs offabrication.

Additional objects, advantages and novel features of the presentinvention will be set forth in part in the description that follows, andin part will become apparent to those skilled in the art uponexamination of the following or may be learned by practice of theinvention. The objects and advantages of the invention may be realizedand attained by means of the instrumentalities and combinationsparticularly pointed out in the appended claims.

According to a first aspect of the present invention, a microstructureincludes a mass; a base member in which the mass is movably contained.The mass includes a surface, which is exposed out of the base membersand a stopper wire, which is arranged above the surface of the mass soas to inhibit over move of the mass. As compared to a conventionaltechnology using a glass stopper, a microstructure of the presentinvention can be fabricated to have a smaller thickness. When stopperwires are fixed, no stress is applied to the base member and the mass.As a result, no negative affection to characteristics of themicrostructure, such as an accelerometer, is made. Further, the stopperwires can be fixed in a wire-bonding process, so that fabricationprocess can be prevented from being complicated; and therefore,fabrication costs may be minimized. Especially, for an accelerometer,the stopper wires can be formed at the same time when electrode pads ofa sensor chip and lead pads of a package are wire bonded for electricalconnection.

According to a second aspect of the present invention, a packagestructure, containing the above-described microstructure, includes atleast one lead pad used for electrical connection with themicrostructure. At least one end of the stopper wire is connected to thelead pad. The stopper wires can be arranged freely and be changed easilyin design. When one end of the stopper wire is connected to an electrodepad of the base member and the other end is connected to a lead pad ofthe package, the stopper wire can be used not only for mechanicalprotection but also for electrical connection.

According to a third aspect of the present invention, the stopper wireincludes ends to be fixed at positions which are relatively lower inlevel than the surface of the mass. It is difficult to arrange a bondingwire having a height or clearance. If a clearance of a bonding wire isless than 100 μm, the clearance would be made irregularly. According tothe present invention, a distance or clearance between an upper surfaceof a mass and a stopper wire can be decreased without increasing adistance in vertical direction between ends and top of the wire. As aresult, a thickness of a microstructure can be reduced withoutincreasing irregularity of clearance or heights of stopper wires.

According to a fourth aspect of the present invention, an accelerometerincludes a mass; a silicon base member in which the mass is movablycontained, wherein the mass comprises a surface which is exposed out ofthe base member; a stopper wire which is arranged above the surface ofthe mass so as to inhibit over move of the mass; and a package whichcontains the base member with the mass.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view illustrating an accelerometer according toa first preferred embodiment of the present invention.

FIG. 2 is a plane view showing the accelerometer shown in FIG. 1, inwhich small elements like metal interconnection are omitted.

FIG. 3 is a cross-sectional view taken on line I-I in FIG. 1.

FIG. 4 is a cross-sectional view showing a part of the accelerometershown in FIG. 1.

FIG. 5 is a plane view showing another arrangement of stopper wires usedfor an accelerometer according to the first preferred embodiment.

FIG. 6 is a plane view showing another arrangement of stopper wires usedfor an accelerometer according to the first preferred embodiment.

FIG. 7 is a cross-sectional view showing a package structure containingand an accelerometer according to a second preferred embodiment of thepresent invention.

FIG. 8 is a plane view showing an arrangement of stopper wires used foran accelerometer according to the second preferred embodiment.

FIGS. 9-12 are plane views showing other arrangements of stopper wiresused for the second preferred embodiment, shown in FIG. 7.

FIG. 13 is a cross-sectional view showing a package structure containingand an accelerometer according to a third preferred embodiment of thepresent invention.

FIG. 14 is a plane view showing an arrangement of stopper wires used foran accelerometer according to the third preferred embodiment.

FIG. 15 is a plane view showing another arrangement of stopper wiresused for an accelerometer according to the third preferred embodiment.

FIG. 16 is a cross-sectional view showing an accelerometer according toa fourth preferred embodiment of the present invention.

FIG. 17 is a perspective view illustrating an accelerometer according toa fifth preferred embodiment of the present invention.

FIG. 18 is a plane view showing an arrangement of stopper wires used foran accelerometer according to the fifth preferred embodiment.

FIG. 19 is a cross-sectional view illustrating an accelerometeraccording to a sixth preferred embodiment of the present invention.

FIGS. 20A-20D are cross-sectional views showing one example offabricating steps of the accelerometer according to the sixth preferredembodiment, shown in FIG. 19.

FIGS. 21A-21G are cross-sectional views showing another example offabricating steps of the accelerometer according to the sixth preferredembodiment, shown in FIG. 19.

FIGS. 22A-22F are cross-sectional views showing another example offabrication steps of the accelerometer according to the sixth preferredembodiment, shown in FIG. 19.

FIG. 23 is a cross-sectional view illustrating an accelerometeraccording to a seventh preferred embodiment of the present invention.

FIGS. 24A-24E are cross-sectional views showing an example offabrication steps of the accelerometer according to the seventhpreferred embodiment, shown in FIG. 23.

FIG. 25 is a perspective view illustrating an accelerometer according toan eighth preferred embodiment of the present invention.

FIG. 26 is a cross-sectional view illustrating an accelerometeraccording to the eighth preferred embodiment, shown in FIG. 25.

BEST MODE FOR CARRYING OUT THE INVENTION

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings which form a part hereof,and in which is shown by way of illustration specific preferredembodiments in which the inventions may be practiced. These preferredembodiments are described in sufficient detail to enable those skilledin the art to practice the invention, and it is to be understood thatother preferred embodiments may be utilized and that logical, mechanicaland electrical changes may be made without departing from the spirit andscope of the present inventions. The following detailed description is,therefore, not to be taken in a limiting sense, and scope of the presentinventions is defined only by the appended claims.

Now the present invention is described. The present invention isapplicable to a variety of types of inertial sensor, such as anaccelerometer, and an angular accelerometer (vibration gyroscope). Thepresent invention is also applicable to any kinds of microstructure(MEMS) having a movable member, such as an actuator.

FIGS. 1 and 2 show an accelerometer 10 according to a first preferredembodiment of the present invention. In FIG. 2, small parts, such asmetal interconnection, are not shown for better understanding of overallof the accelerometer 10. FIG. 3 shows an inside structure of theaccelerometer 10. The accelerometer 10 includes a silicon base member12, and a movable mass 14, which is contained in a cavity of the siliconbase member 12. The mass 14 is provided so as to be able to moveup-and-down and right-and-left, namely in three-dimensional directions.The silicon base member 12 is proved at its center with a squire shapecavity, in which the mass 14 is contained. The movable mass 14 is shapedto be a cloverleaf having four square regions, which are connected atthe center thereof, in order to increase inertia force. An upper surfaceof the movable mass 14 and an upper surface of the base member 12 arearranged in the same level.

The accelerometer 10 further includes four beams 16, which connect themass 14 and base member 12; and eight of piezoresistance elements 18.The piezoresistance elements 18 are arranged at the boundaries betweenthe mass 14 and the beams 16, and between the base member 12 and thebeams 16. Each of the beams 16 is arranged at a gap formed between twoadjacent square pats of the mass 14. The silicon base member 12 isprovided at the upper surface with electrode pads, which are connectedto the piezoresistance elements 18 with a metal interconnection (notshown).

Electrode pads 20, which are not connected to the piezoresistanceelements 18 are connected to ends of stopper wires 22. Four of the wires22 are arranged above all the corners of the movable mass 14. Each wire22 is arranged to extend across a corner of the movable mass 14. Theends of the wire 22 are fixed to the electrode pads 20 by a conventionalwire-bonding process.

As shown in FIG. 3, the silicon base member 12 is fixed on to a die bondsurface 24. Downward over move of the movable mass 14 is inhibited bythe die bond surface 24. Horizontal over-move of the mass 14 isinhibited by inner walls of the silicon base member 12. Upward over-moveof the mass 14 is inhibited by the wires 22. The clearance between themass 14 and the wires 22 can be adjusted by controlling the wire bondingdevice. “Over-move” means movement resulting that the accelerometer 10does not work. For example, if the mass 14 over moves, the accelerometer10 would be broken or would output a signal at a level over its maximumrated level.

FIG. 4 shows a structure around an end of the wire 22 connected to thepad 20. According to the present invention, connecting regions betweenthe wire 22 and the electrode pad 20 can be covered with a resin 28. Asa result, reliability of connection between the wire 22 and the pad 20would be increased.

In fabrication of the above-described accelerometer 10, a SOI wafer isformed from a silicon layer (Si), a buried oxide layer (SiO2) and a Sisubstrate. A bridge circuit is formed on the SOI wafer using asemiconductor technology to form piezoresistance elements 18, a metalcircuit pattern and electrode pads 20. After that, the surface iscovered with a passivating film, such as SiN layer, except the electrodepads 20. Next, beams 16 are formed in a Si Deep RIE (Reactive IonEtching) process. After that, the movable mass 14 is formed in a Si DeepRIE process carried out from the Si substrate. Next, the movable mass 14is released from the substrate in an etching process to the buried oxidelayer. After that, the substrate is cut to form individual sensor chipsin a dicing process. Next, the sensor chip is bonded in a package, then,the electrode pads 20 of the sensor chip 10 and lead pads of the packageare wire bonded for electrical connection. At the same time, the wires22 are formed over the sensor chip 10.

FIGS. 5 and 6 shows example of arrangement of the stopper wire (22).According to FIG. 5, two of wires 122 are arranged in parallel to sidesof the silicon base member 12. Those two wires 122 have the same length.According to FIG. 6, three of wires 222 a and 222 b are arranged inparallel to a diagonal line of the silicon base member 12. A longer wire222 a extends along a diagonal line of the silicon base member 12.Shorter wires 222 b are extend to across over corners of the movablemass 14, in the same manner as shown in FIG. 1. In FIG. 6, the shorterwires 222 b can be omitted, because the longer wire 222 a would be ableto inhibit over-move of the mass 14 in good balance of level.

FIG. 7 is a cross-sectional view showing a package structure containingand an accelerometer 10 according to a second preferred embodiment ofthe present invention. In FIG. 7, the same or corresponding elements tothose in the first preferred embodiment, are represented by the samereference number, and the same description is not repeated here in thisembodiment. According to the second preferred embodiment, both ends ofstopper wire 22 are fixed on to lead pads 32 of a package 30. Ascompared to the first preferred embodiment, the wire 22 can be arrangedwith a high degree of freedom.

FIG. 8-12 are plane views showing examples of arrangement of stopperwires 22 used for the second preferred embodiment, shown in FIG. 7.According to a case shown in FIG. 8, two of wires 22 are arranged toextend in parallel to sides of the accelerometer 10. According to a caseshown in FIG. 9, four of wires 22 are arranged to extend across thecorners of the accelerometer 10, in which none of the wires are crossedto each other. According to a case shown in FIG. 10, four of wires 22are arranged to form a shape of “#” above the accelerometer 10.According to a case shown in FIG. 11, two shorter wires 22 and twolonger wires 22 are arranged to extend in parallel to a diagonal line ofthe accelerometer 10. According to a case shown in FIG. 12, four ofwires 22 are arranged to form a diamond shape, in which each wire 22extend across a corer of the accelerometer 10.

FIG. 13 is a cross-sectional view showing a package structure containingand an accelerometer according to a third preferred embodiment of thepresent invention. In FIG. 13, the same or corresponding elements tothose in the first and second preferred embodiment, are represented bythe same reference number, and the same description is not repeated herein this embodiment.

According to the third preferred embodiment, one end of a stopper wire22 is bonded onto a lead pad 32 of a package 30, and the other end isbonded on to an electrode pad 20 of an accelerometer 10. The stopperwires 22 can be used for electrical connection between the package 30and the sensor 10. The lead pad 32 and electrode pad 20 can be formedfor exclusive use for the stopper wires 22, so that the stopper wires 22can be electrically isolated from the sensor 10. As a result, thestopper wires 22 can be arranged without taking into consideration of awiring design of the sensor 10.

FIGS. 14 and 15 are plane views showing examples of arrangement of thestopper wires 22 used for the accelerometer 10 according to the thirdpreferred embodiment. In a practical example shown in FIG. 14, four ofwires 22 are arranged to form a shape of “#” above the accelerometer 10.In a practical example shown in FIG. 15, four of wires 22 are arrangedto form a diamond shape, in which each wire 22 extend across a corer ofthe accelerometer 10.

FIG. 16 is a cross-sectional view showing an accelerometer according toa fourth preferred embodiment of the present invention. In FIG. 16, thesame or corresponding elements to those in the first to third preferredembodiments are represented by the same reference number, and the samedescription is not repeated here in this embodiment. A feature of thisembodiment is that stopper wires 22 are provided to have the sameelectrical potential level with an SOI wafer 50, so that the wires 22 donot operate as an antenna, which affects negatively to characteristicsof the accelerometer 10.

In fabrication of the above-described accelerometer 10, a SOI wafer isformed from a silicon layer (Si), a buried oxide layer (SiO2) and a Sisubstrate. A bridge circuit is formed on the silicon layer in anion-implantation or thermal diffusion process to form piezoresistanceelements 18. After that, an insulating layer 52 is formed in a thermaloxidation process. Next, contact holes are formed so as that anelectrode pad 20, which is connected to the wire 22, becomes the sameelectrical potential level as the silicon layer (Si). After that, ametal circuit pattern and electrode pads 20 are formed, and the surfaceis covered with a passivating film 54, such as SiN layer, except theelectrode pads 20.

Next, beams 16 are formed in a Si Deep RIE (Reactive Ion Etching)process. After that, the movable mass 14 is formed in a Si Deep RIEprocess carried out from the Si substrate. Next, the movable mass 14 isreleased from the substrate in an etching process to the buried oxidelayer. After that, the substrate is cut to form individual sensor chipsin a dicing process. Next, the sensor chip 10 is bonded in a package,then, the electrode pads 20 of the sensor chip-10 and lead pads of thepackage are wire bonded for electrical connection. At the same time, thewires 22 are formed over the sensor chip 10.

FIGS. 17 and 18 show an accelerometer 10 according to a fifth preferredembodiment of the present invention. In FIG. 18, small parts, such asmetal interconnection, are not shown for better understanding of overallof the accelerometer 10. FIG. 19 shows an inside structure of theaccelerometer 10. The accelerometer 10 includes a silicon base member12, and a movable mass 14, which is contained in a cavity of the siliconbase member 12. The mass 14 is provided so as to be able to moveup-and-down and right-and-left, namely in three-dimensional directions.The silicon base member 12 is proved at its center with a squire shapecavity, in which the mass 14 is contained. The movable mass 14 is shapedto be a cloverleaf having four square regions, which are connected atthe center thereof, in order to increase inertia force.

A projected member 14 a is formed on each of the four square regions ofthe mass 14 so that an upper surface of the projected member 14 a is“Δh” higher than an upper surface of the silicon base member 12.

The accelerometer 10 further includes four beams 16, which connect themass 14 and base member 12; and eight of piezoresistance elements 18.The piezoresistance elements 18 are arranged at the boundaries betweenthe mass 14 and the beams 16, and between the base member 12 and thebeams 16. Each of the beams 16 is arranged at a gap formed between twoadjacent square pats of the mass 14. The silicon base member 12 isprovided at the upper surface with electrode pads, which are connectedto the piezoresistance elements 18 with a metal interconnection (notshown).

Electrode pads 20, which are not connected to the piezoresistanceelements 18 are connected to ends of stopper wires 22. Four of the wires22 are arranged above all the corners of the movable mass 14. Each wire22 is arranged to extend across a corner of the movable mass 14. Theends of the wire 22 are fixed to the electrode pads 20 by a conventionalwire-bonding process.

As shown in FIG. 19, the silicon base member 12 is fixed on to a diebond surface 24. Downward over move of the movable mass 14 is inhibitedby the die bond surface 24. Horizontal over move of the mass 14 isinhibited by inner walls of the silicon base member 12. Upward over-moveof the mass 14 is inhibited by the wires 22. The clearance between themass 14 and the wires 22 can be adjusted by controlling the wire bondingdevice.

A distance “H” between the upper surface of the projected member 14 aand the stopper wire 22 can be adjusted by a wire-bonding device (notshown) and the height difference “A h”. According to this embodiment, adistance “H” can be decreased without increasing a distance “H+Δh”. As aresult, a thickness of the accelerometer 10 can be reduced withoutincreasing irregularity of clearance “H” of stopper wires 22. Even if adistance “H” is determined about 80 μm, irregularity of clearance “H” ofstopper wires 22 would not increase remarkably.

FIGS. 20A-20D are cross-sectional views showing one example offabricating steps of the accelerometer according to the sixth preferredembodiment, shown in FIG. 19. In fabrication of the above-describedaccelerometer 10, a SOI wafer 31 is formed from a silicon layer (Si), aburied oxide layer (SiO2) and a Si substrate. A bridge circuit is formedon the silicon layer in a semiconductor process to form piezoresistanceelements 18, a metal circuit pattern and electrode pads 20. A sensorcircuit 33 is formed, as shown in FIG. 20A.

Next, a photosensitive polyimide or resist is formed on to the sensorcircuit 33 using a spin-coating process, and is exposed, developed andbaked to form a projected member 34 (14 a), as shown in FIG. 20B.

Subsequently, beams 16 are formed in a Si Deep RIE (Reactive IonEtching) process. After that, the movable mass 14 is formed in a Si DeepRIE process carried out from the Si substrate 31. Next, the movable mass14 is released from the substrate 31 in an etching process to the buriedoxide layer, as shown in FIG. 20C. After that, the substrate 31 is cutto form individual sensor chips in a dicing process. Next, the sensorchip is bonded in a package, then, the electrode pads 20 of the sensorchip 10 and lead pads of the package are wire bonded for electricalconnection. At the same time, the wires 22 are formed over the sensorchip 10.

FIGS. 21A-21G are cross-sectional views showing another example offabricating steps of the accelerometer 10 according to the sixthpreferred embodiment, shown in FIG. 19. In fabrication of theabove-described accelerometer 10, a SOI wafer 31 is formed from asilicon layer (Si), a buried oxide layer (SiO2) and a Si substrate. Abridge circuit is formed on the silicon layer in a semiconductor processto form piezoresistance elements 18, a metal circuit pattern andelectrode pads 20. A sensor circuit 33 is formed, as shown in FIG. 21A.

Next, a seed layer 40 is formed over the SOI wafer 31 in a sputteringprocess, as shown in FIG. 21B. The seed layer 40 may be of Ni, Cu, Au,Pd, Ag, Sn, Co or the like. After that, photosensitive polyimide orresist is formed on to the seed layer 40 in a spin-coating process, andis exposed, developed and baked to form a resist pattern 42, as shown inFIG. 21C. Next, a plating layer 40 a is formed from the seed layer 40,as shown in FIG. 21D. After that, the resist pattern 42 is removed, andthe seed layer 40 is removed in an ion-milling process, wet etchingprocess or RIE (Reactive Ion Etching) process to form a projected member44, as shown in FIG. 21E.

Subsequently, beams 16 are formed in a Si Deep RIE (Reactive IonEtching) process. After that, the movable mass 14 is formed in a Si DeepRIE process carried out from the Si substrate 31. Next, the movable mass14 is released from the substrate 31 in an etching process to the buriedoxide layer, as shown in FIG. 21F. After that, the substrate 31 is cutto form individual sensor chips in a dicing process. Next, the sensorchip is bonded in a package, then, the electrode pads 20 of the sensorchip 10 and lead pads of the package are wire bonded for electricalconnection. At the same time, the wires 22 are formed over the sensorchip 10, as shown in FIG. 21G.

FIGS. 22A-22F are cross-sectional views showing another example offabrication steps of the accelerometer 10 according to the sixthpreferred embodiment, shown in FIG. 19. In fabrication of theabove-described accelerometer 10, a SOI wafer 31 is formed from asilicon layer (Si), a buried oxide layer (SiO2) and a Si substrate.Next, a polysilicon layer 48 is formed on the SOI wafer 31, as shown inFIG. 22A. Subsequently, a photosensitive polyimide or resist is formedon to the polysilicon layer 48 in a spin-coating process, and isexposed, developed and baked to form a resist pattern 51, as shown inFIG. 22B.

Next, the polysilicon layer 48 is etched in a RIE process to form aprojected member 53, as shown in FIG. 22C. Subsequently, a bridgecircuit is formed in a semiconductor process to form piezoresistanceelements 18, a metal circuit pattern and electrode pads 20. A sensorcircuit 33 is formed over the projected member 53, as shown in FIG. 22D.After that, the surface is covered with a passivating film, such as SiNlayer, except the electrode pads 20.

Subsequently, beams 16 are formed in a Si Deep RIE (Reactive IonEtching) process. After that, the movable mass 14 is formed in a Si DeepRIE process carried out from the Si substrate 31. Next, the movable mass14 is released from the substrate 31 in an etching process to the buriedoxide layer, as shown in FIG. 22E. After that, the substrate 31 is cutto form individual sensor chips in a dicing process. Next, the sensorchip is bonded in a package, then, the electrode pads 20 of the sensorchip 10 and lead pads of the package are wire bonded for electricalconnection. At the same time, the wires 22 are formed over the sensorchip 10, as shown in FIG. 22F.

FIG. 23 is a cross-sectional view illustrating an accelerometeraccording to a seventh preferred embodiment of the present invention.FIGS. 24A-24E are cross-sectional views showing an example offabrication steps of the accelerometer according to the seventhpreferred embodiment, shown in FIG. 23. In FIGS. 23 and 24A-24E, thesame or corresponding elements to those in the above-described sixthpreferred embodiment are represented by the same reference numerals, andthe same description is not repeated here in this embodiment. Thedifference from the six preferred embodiment is that structure of amovable mass 14 and a silicon base member 58. In this embodiment, themass 14 is not provided with any projected member (14 a), but an uppersurface of the silicon base member 58 is cut out in order to form alevel difference “Δh” between the upper surface of the base member 58and an upper surface of the mass 14.

In fabrication of the above-described accelerometer, a SOI wafer 31 isformed from a silicon layer (Si), a buried oxide layer (SiO2) and a Sisubstrate. Next, a resist is formed on to the SOI wafer 31 in aspin-coating process, and is exposed, developed and baked to form aresist pattern 60, as shown in FIG. 24A. After that, the SOI wafer 31 isetched in a RIE process to form recess portions 31 a, which are used aselectrode pads, as shown in FIG. 24B.

Next, a bridge circuit is formed in a semiconductor process to formpiezoresistance elements 18, a metal circuit pattern and electrode pads20. A sensor circuit 33 is formed on the SOI wafer 31, as shown in FIG.24C. After that, the surface of the substrate 31 is covered with apassivating film, such as SiN layer, except the electrode pads 31 a.

Subsequently, beams 16 are formed in a Si Deep RIE (Reactive IonEtching) process. After that, the movable mass 14 is formed in a Si DeepRIE process carried out from the Si substrate 31. Next, the movable mass14 is released from the substrate 31 in an etching process to the buriedoxide layer, as shown in FIG. 24D. After that, the substrate 31 is cutto form individual sensor chips in a dicing process. Next, the sensorchip is bonded in a package, then, the electrode pads 31 a of the sensorchip 10 and lead pads of the package are wire bonded for electricalconnection. At the same time, the wires 22 are formed over the sensorchip 10, as shown in FIG. 24E.

FIG. 25 is a perspective view illustrating an accelerometer 74 accordingto an eighth preferred embodiment of the present invention. FIG. 26 is across-sectional view illustrating the accelerometer 74, shown in FIG.25. In FIGS. 25 and 26, the same or corresponding elements to those inthe above-described embodiments are represented by the same referencenumerals, and the same description is not repeated here in thisembodiment.

The accelerometer 74 includes a silicon base member 12, and a movablemass 14, which is contained in a cavity of the silicon base member 12.The mass 14 is provided so as to be able to move up-and-down andright-and-left, namely in three-dimensional-directions. The silicon basemember 12 is proved at its center with a squire shape cavity, in whichthe mass 14 is contained. The movable mass 14 is shaped to be acloverleaf having four square regions, which are connected at the centerthereof, in order to increase inertia force. An upper surface of themovable mass 14 and an upper surface of the base member 12 are arrangedin the same level.

The accelerometer 74 further includes four beams 16, which connect themass 14 and base member 12; and eight of piezoresistance elements 18.The piezoresistance elements 18 are arranged at the boundaries betweenthe mass 14 and the beams 16, and between the base member 12 and thebeams 16. Each of the beams 16 is arranged at a gap formed between twoadjacent square pats of the mass 14. The silicon base member 12 isprovided at the upper surface with electrode pads 20, which areconnected to the piezoresistance elements 18 with a metalinterconnection (not shown).

The accelerometer 74 is contained in a package 70, shown in FIG. 26. Thepackage 70 includes recess portions 75 and lead pads 74, which areformed on the recess portions 75. The recess portions 75 and lead pads76 are designed so that an upper surface of the lead pad 76 is “A h”lower than an upper surface of the accelerometer (sensor) 74. Two leadpads, arranged at the opposite sides of the accelerometer 74, areconnected by a stopper wire 22. According to the eight preferredembodiment, the same advantages can be obtained as the above-describedsixth and seventh preferred embodiments.

According to the sixth to eighth preferred embodiments, the stopper wire22 includes ends to be fixed at positions, which are relatively lower inlevel than the surface of the mass 14. A distance “H” between an uppersurface of the mass 14 and the stopper wire 22 can be decreased withoutincreasing a distance (H+Δh) in vertical direction between ends and topof the wire 22. As a result, a thickness of a microstructure can bereduced without increasing irregularity of clearance or heights ofstopper wires 22.

1. A microstructure having a movable mass, comprising: a mass; a basemember in which the mass is movably contained, wherein the masscomprises a surface which is exposed out of the base member, and astopper wire which is arranged above the surface of the mass so as toinhibit over move of the mass.
 2. A microstructure according to claim 1,wherein the stopper wire is connected only to the base member but not tothe mass.
 3. A microstructure according to claim 1, wherein the stopperwire is fixed in a wire-bonding process.
 4. A microstructure accordingto claim 3, wherein ends of the stopper wire connected to the basemember are covered with a resin.
 5. A microstructure according to claim2, wherein the base member comprises pads exclusively used to beconnected with the stopper wire.
 6. A microstructure according to claim2, wherein the base member and the stopper wire are controlled to havean identical electrical potential to each other.
 7. A package structurecontaining a microstructure according to claim 1, comprising: at leastone lead pad used for electrical connection with the microstructure,wherein at least one end of the stopper wire is connected to the leadpad.
 8. A package structure according to claim 7, wherein both ends ofthe stopper wire is connected to the lead pads.
 9. A package structureaccording to claim 7, wherein one end of the stopper wire is connectedto the lead pad, and the other end is connected to an electrode pad ofthe base member.
 10. A package structure according to claim 7, whereinthe lead pad is exclusively used to be connected with the stopper wire.11. A microstructure according to claim 1, wherein the stopper wirecomprises ends to be fixed at positions which are relatively lower inlevel than the surface of the mass.
 12. A microstructure according toclaim 11, wherein a projecting portion is formed on the surface of themass.
 13. A microstructure according to claim 12, wherein the projectingportion is a resin layer formed on the surface of the mass.
 14. Amicrostructure according to claim 12, wherein the projecting portion isa plating layer formed on the surface of the mass.
 15. A microstructureaccording to claim 12, wherein the projecting portion is a polysiliconlayer formed on the surface of the mass.
 16. A microstructure accordingto claim 11, wherein the base member is provided with wire-fixingregions which are connected with the ends of the stopper wire, thewire-fixing regions being formed to have surfaces lower in level thanthe surface of the mass.
 17. A microstructure according to claim 16,wherein the wire-fixing regions are formed to shape the base member tobe stepped structures.
 18. An accelerometer which provides an outputsignal in accordance with movement of a mass, comprising: a mass; asilicon base member in which the mass is movably contained, wherein themass comprises a surface which is exposed out of the base member; astopper wire which is arranged above the surface of the mass so as toinhibit over move of the mass; and a package which contains the basemember with the mass.
 19. An accelerometer according to claim 18,wherein the package comprises at least one lead pad used for electricalconnection with the microstructure, wherein at least one end of thestopper wire is connected to the lead pad.
 20. An accelerometeraccording to claim 19, wherein both ends of the stopper wire isconnected to the lead pads.
 21. An accelerometer according to claim 19,wherein one end of the stopper wire is connected to the lead pad, andthe other end is connected to an electrode pad of the base member. 22.An accelerometer according to claim 18, wherein the stopper wirecomprises ends to be fixed at positions which are relatively lower inlevel than the surface of the mass.